In prior art systems, a light set includes a light socket and a light bulb. A hook is then provided on a side of the light socket, as in U.S. Pat. No. 5,051,877, FIG. 9. Every light set is hung on the electric wires by the hook of each light socket. In use, it can be found that each light set is inclined and the light set string looks bad. When a light bulb is broken, it is typically replaced without turning off the electric source. During rotation of the bulb, one may touch metal parts of the bulb and get an electric shock.